Twisted van der Waals heterostructures have become a testing ground for creating exotic quantum phases. Among them, the twisted cuprate BSCCO holds a special place: theory predicts that at an angle of 45° it becomes a topological superconductor with chiral edge modes. Until now, however, these modes had not been observed directly, and the size of the topological gap remained unknown. The work sits within the broader study of twisted superconductivity and moiré topology, alongside research on twisted graphene and other layered systems, but it is the first to apply quantum sensing to cuprate heterostructures.
The authors fabricated devices from twisted BSCCO using a cryogenic, solvent-free transfer method. To observe the edge modes, they used a single NV center implanted in a diamond nanopillar structure. The NV center works as a quantum magnetic field sensor: its spin relaxation speeds up under the magnetic noise from quasiparticles in the edge channels. By scanning the NV center over the sample at a distance of ~60 nm, they measured the relaxation rate Γ as a function of position and temperature. To study the field-induced topology, they used gradiometry to reconstruct the current density.
In BSCCO twisted at 45°, at 50 K, two peaks in the relaxation rate Γ appeared exactly at the physical edges of the device, which is direct evidence of chiral edge modes. The dependence of Γ on the distance to the edge is inversely proportional to the square of the distance, in agreement with theory. From the temperature dependence of Γ, the topological gap was estimated as Δ ≈ 4 meV at 0 K. The topological transition occurs at a temperature T̃_c that depends on the twist angle: for 45° it coincides with T_c ≈ 86 K, for 43° it is 70 K, for 40° it is 30 K, and at angles of 37° and 24° the edge modes are absent. Applying a longitudinal magnetic field of 20 G creates a periodic pattern of alternating domains with opposite chirality, which points to the formation of a Josephson vortex lattice.
The work demonstrates the first direct observation of topological edge modes in a twisted cuprate, which sets it apart from previous studies based on transport measurements or theoretical models. The quantum sensing method makes it possible to quantitatively estimate the topological gap and its dependence on the twist angle, opening the way to systematic studies of topology in twisted superconductors. This is an important step toward realizing high-temperature topological superconductivity for quantum computing.
The topic could develop toward a more detailed study of the edge modes: their spatial structure, spin texture, and dynamics. It may be possible to investigate the influence of disorder, layer thickness, and other twist angles. Applying quantum sensing to other twisted systems, such as twisted bilayer graphene or transition metal dichalcogenides, looks promising.
The results will influence condensed matter physics, quantum technologies, and materials science. The scanning quantum magnetometry method will find use in studying a broad class of moiré materials.
Next steps include measuring edge currents with higher spatial resolution, studying the influence of magnetic field and temperature on the chiral domains, and attempts to detect Majorana modes in similar systems.
The work is directly connected to the problem of high-temperature topological superconductivity and the search for Majorana fermions for fault-tolerant quantum computing. It also sheds light on the pairing mechanisms in cuprates and the role of order parameter symmetry.
An NV center in diamond is an atom-sized defect capable of sensing the magnetic field from a single electron spin at distances of tens of nanometers. In this work it played the role of a "quantum ear," picking up the noise of quasiparticles racing along the edge of a superconductor.
The idea of topologically protected edge states echoes the concept of "insulating walls" in science fiction, for example the protective fields in Greg Egan's novel Quarantine, where information is stored in topological defects of spacetime.